The present invention relates in general to well logging instruments of the scintillation detector type, and more particularly to the use of plastic scintillation elements for high-temperature well logging.
Well logging instruments incorporating scintillation elements are well known in the art. U.S. Pats. Nos. 4,004,151; 4,158,773; and 4,360,733, owned by the assignee of the present invention and incorporated herein by reference in their entireties, illustrate such instruments as including an inorganic crystal scintillation element composed of an alkali metal halide. For example, high purity thallium activated sodium iodide crystals have been used successfully as scintillation elements in high-temperature deep well logging applications. Such crystal scintillation elements are inherently mechanically resistant to the high temperatures (150.degree. C.-200.degree. C.) of deep well logging, and do not soften and deform, or otherwise structurally fail, as a sole result of such temperatures.
While many well logging applications utilize crystal scintillation elements, some applications could benefit from the use of organic, noncrystalline, amorphous scintillation elements, such as plastic scintillation elements, due to their high shock resistance and generally lower cost as compared to crystal scintillation elements. For example, polystyrenebased plastic scintillators sold by the assignee of the present invention under the trade designations BC-434 and BC-438 could be utilized.
The immediate problem presented by the use of such plastic scintillation elements in high-temperature well logging is their low-temperature resistance as compared to the earlier-noted inorganic crystals. Plastic scintillators, when subjected to temperatures in excess of for example 100.degree. C., can soften and deform so as to lose their shape, resulting in failure of the scintillation element. Also, due to the softening and deformation problem, it becomes more difficult to provide and maintain efficient optical coupling between the plastic scintillation element and an associated photomultiplier tube.
It is a goal of the present invention to provide a highly reliable, high-temperature, deep well logging instrument incorporating a plastic scintillation element susceptible to softening and deformation at the operating temperatures of the instrument.